We are interested in improving the efficiency of retroviral mediated gene transfer. Retroviral vectors provide the most efficient means of stably expressing genes in cells both in vitro and in vivo. However not all types of cells are efficiently infected by retroviruses. We have developed an approach that allows us to infect cells that are refractory to retroviral infection. This approach is predicated on identifying and functionally characterizing the receptors used by retroviral vectors to gain entry into cells and mapping the regions of the cellular receptor which are critical for retrovirus-receptor interaction. We have used three different types of recombinant vectors, two murine based vectors (ecotropic and amphotropic) and a gibbon ape vector (GaLV) in our analyses. 1. We have cloned a murine ecotropic receptor and precisely mapped the amino acid residues that are critical for binding and entry of ecotropic vectors. 2. A segment of human DNA which confers susceptibility to amphotropic virus infection (a putative amphotropic receptor) has been functionally identified and is in the process of being isolated and molecularly characterized. 3. We have used oligomeric probes derived from the GaLV receptor sequence to examine spatial and temporal expression of receptor mRNAs in the rat by in situ hybridization. If the GaLV receptor is restricted to a certain cell type, it may be possible to target genes using GaLV retroviral vectors to a particular tissue. We have also initiated studies to functionally map the GaLV binding region of the receptor.